Signal compressors and expanders

ABSTRACT

CHARACTERISTIC SAID SECOND FILTER INCLUDING A VARIABLE IMPEDANCE MEANS RESPONSIVE TO AN INCREASE IN THE AMPLITUDE OF AT LEAST ONE OF THE OUTPUT OF AND THE INPUT TO SAID FURTHER SIGNAL PATH FOR ALTERING SAID CUTOFF CHARACTERISTICS FROM WIDE BAND PASS UNDER VERY LOW INPUT SIGNAL CONDITIONS TO PROGRESSIVELY NARROWER BAND PASS, PROVIDING A CUTOFF FREQUENCY WHICH SHIFTS AWAY FROM SAID SECOND LIMIT SUFFICIENTLY TO ATTENUATE, IN COMBINATION WITH THE SAID CHARACTERISTICS OF SAID FIRST FILTER, ANY HIGH-LEVEL INPUT SIGNAL COMPONENTS AT ANY GIVEN FREQUENCY IN SAID SPECIFIED FREQUENCY BAND TO A SPECIFIED LEVEL CORRESPONDING IN SAID SECOND SIGNAL TO A SMALL FRACTIONAL PART OF THE MAXIMUM LEVEL OF SAID INPUT SIGNAL, WHILE PERMITTING INPUT SIGNAL COMPONENTS BELOW SAID SPECIFIED LEVEL AT FREQUENCIES NEARER TO SAID FIRST LIMIT THEN SAID GIVEN FREQUENCY TO PASS THROUGH SAID FILTERS AND TO APPEAR IN SAID SECOND SIGNAL WITH A LESSER DEGREE OF ATTENUATION. 1. A SIGNAL PROCESSING SYSTEM FOR PRODUCING AN OUTPUT SIGNAL IN RESPONSE TO AN INPUT SIGNAL COMPRISING A MAIN SIGNAL PATH RESPONSIVE TO SAID INPUT SIGNAL AND INCLUDING MEANS FOR PROVIDING IN A SPECIFIED FREQUENCY BAND EXTENDING BETWEEN FIRST AND SECOND LIMITS A FIRST SIGNAL SUBSTANTIALLY PROPORTIONAL TO SAID INPUT SIGNAL, A FURTHER SIGNAL PATH COUPLED TO SAID MAIN SIGNAL PATH AND RESPONSIVE TO A SIGNAL DERIVED FROM SAID MAIN SIGNAL PATH FOR PRODUCING A SECOND SIGNAL IN SAID SPECIFIED FREQUENCY BAND, AND SIGNAL COMBINING MEANS FOR COMBINING SAID FIRST AND SECOND SIGNALS TO PRODUCE SAID OUTPUT SIGNAL, SAID FURTHER SIGNAL PATH INCLUDING A FIRST FILTER HAVING FIXED VALUE COMPONENTS FOR PROVIDING PASS CHARACTERISTICS IN A FIXED BAND EXTENDING FROM SAID FIRST LIMIT TO A CUTOFF FREQUENCY INTERMEDIATE SAID FIRST LIMIT AND SAID SECOND LIMIT; AND MEANS FOR LIMITING THE AMPLITUDE OF THE OUTPUT SIGNAL OF THE FURTHER SIGNAL PATH, SAID MEANS INCLUDING A SECOND FILTER CONNECTED IN CASCADE WITH SAID FIRST FILTER AND HAVING VARIABLE CUTOFF

May 20, 1975 R. M. DOLBY Re. 28,426

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srerm, COIPRE'SSORS AND nxrmnnns Original Filed 001;. 20, 1969 8Sheets-Sheet 8 N N N I I N M Q Q Q I t Q Q Q Q $3 Q a E & N Q m R M Q mB 8 9 LI. N g N x A I 9 8 a D g \5 g f rods United States Patent I Re.28,426 Reissuecl May 20, 19-75 28,426 SIGNAL CGMPRESSORS AND EXPANDERSRay Milton Dolby, London, England, assign-or to Dolby Laboratories,Inc., New York, N.Y.

Original No. 3,631,365, dated Dec. 28, 1971, Ser. No. 867,454, Oct. 20,1969. Application for reissue Dec. 27, 1973, Ser. No. 428,914

Claims priority, application Great Britain, July 21, 1967,

36,466/69; Nov. 1, 1963, 51,985/68 Int. Cl. H03q 7/06; H04b 3/04 US. Cl.333-14 45 Ciaims Matter enclosed in heavy brackets [II appears in theoriginal patent but forms no part of this reissue specification; matterprinted in italics indicates the additions made by reissue.

ABSTRACT OF THE DISCLOSURE Signal compressor or expander action isobtained by passing the signal through a main path which provides anundistorted signal. A further path is tapped off the main path andamplifies and limits the signal. The output of the further path is usedto boost or buck the signal in the main path to obtain compressor orexpander action. This action takes place in a restricted frequency banddefined by a variable filter in the further path, this filter beingresponsive to the output of the further path to narrow the pass bandwhen this output increases. The filter comprises two filters incascade-a fixed value filter and a variable filter which includes avoltage-controlled variable resistance. The two filters can be simple RCfilters but in combination they give a 12 db./octave cut-off which isimportant in avoiding noise modulation effects. Various advantagesfilter configurations are disclosed.

INTRODUCTION This invention relates to signal compressors and ex panderssuch as are disclosed in the specification of my co-pending applicationsSer. No. 569,615 and a continuation-in-part thereof, Ser. No. 789,703.The invention is applicable to both Type I and Type II devices, asdefined in Ser. No. 789,703 and concerns further improvements in devicesof the nature disclosed in FIG. 11 of Ser. No. 569,615 and FIG. 23 ofSer. No. 789,703.

The main characteristic of all the devices described in both theabovementioned specifications is that no attempt is made to establishthe required compression or expansion law by operating upon the wholedynamic range of the signal. Rather a main, straight-through signal pathis provided, through which signals, and in particular high levelsignals, can pass undistorted. With these signals is combined the outputof a further path, which can take its input either from the input to orthe output from the device. This output, at low signal levels, eitherboosts or bucks the main signal to provide compression or expansion,respectively. However, the further path includes a limiter so that, athigher signal levels, the output of this path is negligible comparedwith the main signal, resulting in minimal boosting or bucking. In thisWay, a compression or expansion characteristic is derived withsubstantial avoidance of the severe problems inherent in previouslyknown devices which operate on the whole signal in accordance with anon-linear law. It is particularly important that compressors andexpanders according to the invention can be made truly complementary, sothat a complete noise reduction system can be provided which does notitself introduce distortion.

For convenience, idealised block diagrams of Type I and Type II devicesare shown in FIGS. 1 and 2 of the accompanying drawings.

It is well known that noise reduction is desirably cffected selectivelythroughout a signal band, e.g. an audio band, usually in an upper bandand/or a lower band of the overall signal band. The aforementioned FIG.11 concerns the use of a further path with a variable cut-off filterwhich restricts the compression or expansion to an extreme (i.e. highpass or low pass) band of the signal band. As the output amplitude ofthe signal passed by the filter increases, the cut-off frequency isautomatically shifted nearer to the relevant extreme of the signal band;the pass band of the filter is thereby narrowed, so allowing noisereduction still to take place within this narrowed hand without beinginfluenced by the larger amplitude signals excluded by the shift of thefilter cut-off frequency.

THE INVENTION The present invention is concerned with an improved filterconfiguration in the further path enabling suitable variable cut-offcharacteristics to be obtained while requiring only a single voltagecontrolled variable resistance device for varying the cut-off frequency.

The present invention provides a signal compressor of expandercomprising a straight-through signal path and means for combining withthe signals therein, so as either to boost or buck such signals, theoutput signal of a further path which takes its input either from theinput to or the output from the compressor or expander and whichincludes both means for limiting the amplitude of the said output signaland a variable cut-off filter for restricting signals passing throughthe further path to a restricted band extending to one limit of theoverall signal band, the variable filter being responsive to theamplitude of the output thereof to shift the cut-off frequency nearer tothe said one limit of the signal band as the said amplitude increases,the filter configuration comprising two filters in cascade, namely afirst filter comprising fixedvalue components and a second filter with avariable cutoff frequency which, under quiescent conditions, liesfurther from the said one limit than the cut-01f frequency of the firstfilter, the second filter including a voltage controlled variableresistance device responsive to the said amplitude to vary theparameters of the filter and shift the variable cut-off frequencytowards the said one limit.

The variable resistance device is preferably a field effect transistor.

The advantages of the invention will become clear in the followingdescription of detailed embodiments of a compressor and an expander.These embodiments are Type I devices, but essentially the same circuitrycould be employed in the difierent configurations appropriate to Type IIdevices. These embodiments are designed for operation at the upper endonly of the audio signal band, as they are intended for use inconjunction with domestic tape recorders. Thus the said one limit of thesignal band is the upper limit of the band. The invention could equallywell be employed at the lower end of the audio band (the relevant saidone limit being the lower limit of the band) in which case the high passfilters hereinafter described would have to become low-pass filters(with appropriate redesign of the circuit throughout). The invention isfurthermore not limited to audio applications, and it will beappreciated that the following description is throughout by way ofexample.

THE DRAWINGS FIGS. 1 and 2 are the aforementioned diagrams of Type I andII devices,

FIG. 3 is a more detailed block diagram of a Type I sliding bandcompressor,

FIG. 4 is a circuit diagram of a compressor embodying the presentinvention,

FIG. 5 is a circuit diagram of a complementary expander,

FIG. 6 is a partial circuit diagram of a switched compressor/ expander,

FIG. 7 shows a simple passive equalizing network,

FIGS. 8 and 9 show characteristic curves of the compressor,

FIG. 10 is a circuit diagram of an improved filter/ limiter, and

FIGS. 11 to 13 show characteristic curves relating to the embodiment ofFIG. 10.

BACKGROUND FIGS. 1, 2 and 3 all relate generically to the improvementsin the previously mentioned specification as well as to the presentinvention. FIGS. 1 and 2 have already been described. FIG. 3 shows aType I sliding band compressor in more detail; the expander and Type IIvariants will be apparent from FIGS. 1 and 2.

In FIG. 3 the input signal applied to a terminal 1 passes through aresistor 2, constituting the main path, to be summed at an outputterminal 3 with the signal from the further path provided through aresistor 2a. The further path comprises a filter/limiter 4 with input Aand output B, followed by an amplifier 5 and clipper 6, which eliminatesany transient overshoots remaining in the signal passed by theessentially syllabic limiter 4. The syllabic action is obtained byapplying a smoothed control signal to a control terminal C of thefilter/limiter 4. This control signal is derived from the output and/orinput of the further path (lines 7 and 8 of which only one need beprovided) via an amplifier 9, rectifier 11 and smoothing circuit 13. Afilter 8a may also be provided in line 8. When the control signalincreases in amplitude, it narrows the pass band of the filter/limiter4.

A Compressor Embodying the Present Invention The circuit of FIG. 4 isspecifically designed for incorporation in the record channel of adomestic tape recorder, two such circuits being required for a stereorecorder. The input signal is applied at terminal to an emitter followerstage 12 which provides a low impedance signal. This signal is appliedfirstly through a main, straight-through path constituted by a resistor14 to an output terminal 16 and secondly through a further path the lastelement of which is a resistor 18 also connected to the terminal 16. Theresistors 14 and 18 add the outputs of the main and further paths toprovide the required compression law.

The further path consists of a fixed filter 20, a variable cut-offfilter 22 including a PET 24 (these constituting the filter/limiter),and an amplifier 26 the output of which is coupled to a double diodelimiter or clipper 28 and to the resistor 18. The amplifier 26 increasesthe signal in the further path to a level such that the knee in thecharacteristic of the limiter 28, comprising silicon diodes, iseffective at the appropriate signal level under transient conditions.The resistors 14 and 18 are so proportioned that the requiredcompensating degree of attenuation is then provided for the signal inthe further path.

The output of the amplifier 26 is also coupled to an amplifier 30 theoutput of which is rectified by a germanium diode 3-1 and integrated bya smoothing filter 32 to provide the control voltage for the FET 24. Thepoints A, B and C are marked in correspondence with FIG. 3.

The Filter/Limiter Two simple RC filters are used, though equivalent LCor LCR filters could be used. The fixed filter provides a cut-offfrequency of 1700 Hz., below which diminishing compression takes place.The filter 22 comprises a series capacitor 34 and shunt resistor 36followed by a series resistor 38 and the FET 24, with its source-drainpath connected as a shunt resistor. Under quiescent conditions with zerosignal on the gate of the FET 24, the PET is pinched off and presentssubstantially infinite impedance; the presence of the resistor38 canthen be ignored. The cut-off frequency of the filter 22 is thus 800 Hz.,which it will be noted is substantially below the cut-off frequency ofthe fixed filter 20.

When the signal on the gate increases sufficiently for the resistance ofthe FET to fall to less than say 1 K, the resistor 38 effectively shuntsthe resistor 36 and the cut-off frequency rises to 3500 Hz., markedlynarrowing the pass band of the filter. The rise in cut-off frequency isof course a progressive action.

The use of a PET is convenient because, within a suitably restrictedrange of signal amplitudes, such a device acts substantially as a linearresistor (or either polarity signal), the value of which is determinedby the control voltage on the gate.

The use of two cascaded filters (l2 db./octave) is important becausethis results in less noise modulation than a simple one-stage variablecut-off frequency filter (6 db./0ctave). However, the phase delay,varying as a function of frequency, produced by two filters in cascadeis such as to give a compression versus frequency characteristic as isillustrated in FIG. 8 by curve 40 for an input signal 44 db. down on apeak input taken as 0 db., rather than the desired shape of curve 42. Byputting the quiescent cut-off of the filter 22 well below that of thefilter 20 it is possible to achieve a compromise such as curve 44, whichis based on an actual measurement and in which the high frequency end isabout 10 db. up on the low frequency end. Curves for inputs at l6 db.,]0 db., and 0 db. are also shown to illustrate how the compressionaction is progressively lessened as the input amplitude increases andthe band of the filter 22 narrows. In the drawing shown, the curves -areclosed together in the vertical scale more than they are in actuality.

The variable band action is illustrated more clearly by the curves ofFIG. 9, which show the results of putting a strong (0 db.) signal at afixed frequency on the input terminal 10 and superimposing a weak (41db.) signal which is swept through the whole frequency spectrum. Theoutput on terminal 16 at the frequency of the weak signal is detected bya wave analyser and naturally exhibits a strong signal at the frequencyof the fixed frequency strong input and also exhibits the high frequencylift provided by the compression action. It can be clearly seen how theincreasing frequency of the strong signal (200 Hz., 400 Hz., 700 Hz., 1kHz., 2 kHz.) gradually narrows the band within which compression takesplace.

Returning to the description of FIG. 4, the resistor 36 and PET 24 arereturned to an adjustable tap 46 in a potential divider which includes atemperature compensating germanium diode 48. The tap 46 enables thecompression threshold of the filter 22 to be adjusted.

Amplifier and Limiter The amplifier 26 comprises complementarytransistors giving high input impedance and low output impedance. Sincethe amplifier drives the diode limiter 28, a finite output impedance isrequired and is provided by a coupling resistor 50. The diodes 28 are,as already noted, silicon diodes and have a sharp knee around /z volt.

The signal on the limiter and hence on the resistor 18 can be shorted toground by a switch 53 when it is required to switch the compressor outof action.

Control Amplifier and Smoothing Filter The amplifier 30 is an npntransistor with an emitter time constant network 52 giving increasedgain at high frequencies. Strong high frequencies (e.g. a cymbal crash)will therefore lead to rapid narrowing of the band in which compressiontakes place, so as to avoid signal distortion.

The amplifier is coupled to the smoothing filter 32 through therectifying diode 31. The filter comprises a series resistor 54 and shuntcapacitor 56. The resistor 54 is shunted by a silicon diode 58 whichallows rapid charging of the capacitor 56 for fast attack. coupled withgood smoothing under steady-state conditions. The voltage on capacitor56 is applied directly to the gate of the FET 24.

The Complementary Expander A complete circuit diagram is provided inFIG. 5, but a full description is not required as substantially al thecircuit is identical to FIG. 4; component values are there fore not forthe most part shown in FIG. 5. The characteristic curves, although notshown, are complementary to those of FIG. 8.

The differences between FIGS. 4 and 5 are as follows:

In FIG. 5 the further path derives its input from the output terminal16a, the amplifier 26a is inverting, and the signals combined by theresistors 14 and 18 are applied to the input (base) of the emitterfollower 12, the output (emitter) of which is coupled to the terminal16a. To ensure low driving impedance, the input terminal a is coupled tothe resistor 14 through an emitter follower 60. Suitable measures mustbe taken to prevent bias getting into the expander.

The amplifier 26a is rendered inverting by taking the output from theemitter, instead of the collector, of the second (pnp) transistor. Thisalteration involves shifting the 10 K resistor 62 (FIG. 4) from thecollector to the emitter (FIG. 5), which automatically gives a suitableoutput imperance for driving the limiter. The resistor 50 is thereforeomitted in FIG. 5.

It should be noted that it is important in aligning a complete noisereduction system to have equal signal levels on the emitters of thetransistors 12 in both compressor and expander. Metering terminals M areshown connected to these emitters.

Switched Compressor/ Expander In a high quality tape recorder, aseparate compressor and expander can be provided in the record andplayback channels respectively. However, a more economical propositionis to utilize a single compressor/expander with a mode switch forselecting the compressor or expander configuration. The further path forthis compressor/expander can be as shown in FIG. 5i.e. everything to theright of the points X and Y in FIG. 5 is used unchanged, but thecircuitry to the left of points X and Y is changed as shown in FIG. 6.

In FIG. 6 the input signal on terminal 10b is applied to a first emitterfollower stage 70, the output of which is applied through a resistor 72to another emitter follower stage 74 (corresponding to the stage 12 ofFIGS. 4 and 5). The emitter of this stage is connected to the point X toprovide the non-inverted input to the further path. It will be recalledfrom the description of FIG. 5 that, in this embodiment, the furtherpath effects inversion. Accordingly the signal received at the point Y,and applied in the record mode by a mode switch 76 to the outputterminal 16b through the resistor 18, is an inverted signal. To obtaincompressor action, the main path must also effect inversion, which isachieved by means of a further transistor 78 with its base driven fromthe emitter follower 74. The main path signal is taken from a collectorload resistor 80.

In the playback (i.e. expander) configuration, the signal from thefurther path is applied by the switch 76 to the base of the emitterfollower 74, i.e. before the inverter stage 78. The signal from thefurther path therefore combines subtractively with the signal in themain path to give expander action.

Simple Equalization The compressor and expander are provided forincorporation in a high quality tape recorder, permitting the userthereof to record and replay his own tapes with noise reduction. Such atape recorder can obviously also play back pre-recorded tapes which havebeen recorded using the compressor of FIG. 4 or a compressor with likecharacteristics. In order to make it possible for such tapes to bemarketed for universal consumption it is desirable to provide a cheapermeans of equalizing the signal for incorporation in less expensiverecorders. Only by the use of a complementary expander can anundistorted signal be recovered but, to the uncritical ear, the onlynoticeable defect in the compressed signal is an undue emphasis of highfrequencies. This emphasis can be approximately corrected by a simple,passive, treble cut circuit (which acts upon the whole signal). Onesuitable circuit is illustrated in FIG. 7, the component values beingsuitable for equalizing a signal recorded through the compressor of FIG.4.

Further Improvement of the Filter/Limiter In the simplified audio noisereduction system as so far described, it has been pointed out that thefilter/ limiter circuit used represents a compromise. For best noisereduction results, especially with regard to noise modulation effects,it is necessary to use a further path filter which has an attenuationrate of at least 12 db. per octave. The high-pass filter required forhiss reduction is conveniently obtained by the use of the two-sectioncascaded RC network. The last capacitor is paralled by the FET 24 whichcan shift the cut-off frequency of the last section upwards to effectlimiting of the signal.

Unfortunately, the phase shift produced by the twosection filter resultsin a mid-band dip in the compressor frequency response. Thedisadvantages or inconveniences of the dip include an increase in noiselevel in the few hundred hertz region (as opposed to noise reduction)and a disparity in signal level (about 1 db.) in the 1 kHz. region athigh levels when the further path component is switched on and off(noise reduction on-off). An improved filter/limiter circuit is nowdescribed which overcomes the mid-band dip problem and also results inother advantages, notably in decreased dynamic and frequency responseerrors under conditions of imperfect compressor and expander matching.

FIG. 10 shows the improved circuit, for replacing the circuit betweenpoints A, B and C in FIGS. 4 and 5. When the FET 24 is pinched off, thesecond RC network 22 is inoperative, and the first RC network 20 thencontrols the response of the further path. The improved circuit combinesthe phase advantages of having only a single RC section under quiescentconditions with the 12 db. per octave attenuation characteristics of atwo-section RC filter under signal conditions.

In the practical circuit, using MPF 104 FETs, the 39 K resistor 36a isnecessary in order to provide a finite source impedance to work into theFET. In this way the compression ratio (decibel changes in the inputdivided by decibel changes in the compressor output) at all frequenciesand levels is held to a maximum of about 2. The 39 K resistor 36a servesthe same compression ratio limiting function in the improved circuit asthe resistor 36 in the circuit of FIG. 4 or FIG. 5. In addition thisresistor provides a low frequency path for the signal.

FIG. 11 is a chart recording of the input-output response of thecompressor as a function of frequency using the improved filter/limiterof FIG. 10 and the values 47 ohms and 0.1 ,uf. in the emittertime-constant circuit 52 of the control amplifier, instead of the valuesof 220 ohms and 0.15 pf. shown in FIG. 4. FIG. '12 is a plot of theresponse of the circuit below the compression threshold; the expanderresponse is also shown.

It can be seen that with the circuit of FIG. 10 the mid-band dip of FIG.8 is absent. The tendency for the noise reduction system to increase thenoise level in the region of the dip is eliminated, and an improvedoverall noise reduction effect is thereby achieved. The elimination ofthe dip at high levels should also be noted. Thus, when the noisereduction action is switched on and off, there will be no change inlevel and therefore no ambiguities in measuring or specifying levels forstandardization purposes. The high-level dip elimination is broughtabout by the favourable phase characteristics of the improved circuitunder high compression conditions, notably by the provision of a low andmid-frequency path by the resistor 36a. By adjusting the value of the 39K resistor 36a in the second RC network 22 it is possible to achieveeither a dip or a hump in the mid-frequency region.

A further aspect of the improved circuit of FIG. 10 is that it ispossible to achieve greater effective limiting of the further pathcomponent at high frequencies without adversely affecting the matchingcharacteristics. The improved performance is brought about by theleading phase shift introduced at high frequencies by the fully slidingband circuit. In addition, the gain of the control amplifier isincreased at high frequencies. The reduction in threshold and theincreased compression at high frequencies can be seen in FIG. 11. Thecharacteristics shown result in minimal possibility of tape overload atshort wavelengths, although the limiting threshold level isprogressively increased with decreasing frequency in order to reducenoise modulation effects.

A further aspect of the improved circuit also concerns noise modulationeffects. In the circuit of FIG. 4, the variable band action changes tonormal compression when the FET resistance decreases below that of the 1K resistor 38 in series with the FET. The highest turnover frequency ofthe variable filter is high enough to provide acceptably low noisemodulation effects under nomral tape recording noise levels. Cassettes,however, have very high noise levels and it has been found best toeliminate the K resistor 38 and to depend solely on the sliding bandaction to effect limiting. The further path then has better highfrequency transmission in the presence of high amplitude, lowerfrequency signals (which necessarily cause the circuit to operate).

The variable band action of the improved circuit can be seen in FIG. 13,which is produced in the same manner as FIG. 9 by plotting thecompressor frequency response by means of a low-level probe tone (thelevel of which is below the compressor threshold) 'in the presence of ahigh-level signal; the probe is detected at the compressor output bymeans of a tracking filter. The high-level signal causes the compressorcircuitry to operate, the graph showing the effect on the turnoverfrequency of the filter. It can be seen that the variable bandcircuitry, particularly if using the fully sliding configuration, willprovide a significant reduction of high frequency noise under signalconditions. FIGS. 11, 12 and 13 are all taken from actual chartrecordings obtained from the improved circuit of FIG. 10.

A correctly adjusted compressor and expander pair should match at alllevels and frequencies to within about :1 db.; experimental results haveshown this to be attainable at all signal levels. In practice it isimportant that the errors produced under mismatch conditions should notnoticeably affect the quality of reproduction. The errors produced bythe improved filter/limiter circuit of FIG. 10 are less than in thecircuit of FIG. 4. It has been shown experimentally that a 2 db. gain orloss between the compressor and expander results in errors such that thechange in response as a function of frequency reaches a maximum rate ofonly about 2-3 db. per octave, which is low enough to avoid introducingany significant colouration into the reproduced signal.

I claim:

1. A signal processing system for producing an output signal in responseto an input signal comprising a main slgnal path responsive to saidinput signal and including means for providing in a specified frequencyband extending between first and second limits a first signalsubstantially proportional to said input signal, a further signal pathcoupled to said main signal path and responsive to a signal derived fromsaid main signal path for producing a second signal in said specifiedfrequency band, and signal combining means for combining said first andsecond signals to produce said output signal, said further signal pathincluding a first filter having fixed value components for providingpass characteristics in a fixed band extending from said first limit toa cutoff frequency intermediate said first limit and said second limit;and means for limiting the amplitude of the output signal of the furthersignal path, said means including a second filter connected in cascadewith said first filter and having variable cutoff characteristics, saidsecond filter including a variable impedance means responsive to anincrease in the amplitude of at least one of the output of and the inputto said further signal path for altering said cutoff characteristicsfrom wide band pass under very low input signal conditions toprogressively narrower band pass, providing a cutoff frequency whichshifts away from said second limit sufficiently to attenuate, incombination with the said characteristics of said first filter, anyhigh-level input signal components at any given frequency in saidspecified frequency band to a specified level corresponding in saidsecond signal to a small fractional part of the maximum level of saidinput signal, while permitting input signal components below saidspecified level at frequencies nearer to said first limit then saidgiven frequency to pass through said filters and to appear in saidsecond signal with a lesser degree of attenuation.

2. A signal processing system according to claim 1 wherein the variableimpedance means is field effect transistor.

3. A signal processing system according to claim 1 wherein the firstfilter is a single section RC filter.

4. A signal processing system according to claim 1 wherein the secondfilter comprises a series arm consisting of a capacitor in parallel witha resistor and a shunt arm comprising a variable resistance device.

5. A signal processing system according to claim 1 wherein the secondfilter includes a series capacitor followed by a shunt resistor followedby a further shunt arm comprising a variable resistance device.

6. A filter according to claim 4 wherein the variable resistance deviceis a field effect transistor.

7. A filter according to claim 5 wherein the variable resistance deviceis a field effect transistor.

8. A signal processing system according to claim 1 wherein said signalcombining means additively combines said first signal and said secondsignal, whereby said signal processing system operates as a compressor.

9. A signal processing system according to claim 1 wherein said signalcombining means subtractively combines said first and said secondsignals, whereby said signal processing system operates as an expander.

10. A signal processing system according to claim 8 wherein said furthersignal path is responsive to said input signal.

11. A signal processing system according to claim 8 wherein said furthersignal path is responsive to said output signal.

12. A signal processing system according to claim 9 wherein said furthersignal path is responsive to said input signal.

13. A signal processing system according to claim 9 wherein said furthersignal path is responsive to said output signal.

14. A signal processing system according to claim 1 wherein said furthersignal path further comprises a nonlinear limiter in cascade with andfollowing said first and second filters for clipping transients in saidsecond signal.

15. A signal processing system according to claim 1 comprising switchmeans for selecting the compression or expansion operating mode of saidsystem, said switch means selectively effecting signal connectionswhereby for compression mode said further signal path is responsive tosaid input signal and said combining means combines said first andsecond signals additively and whereby for expansion mode said furthersignal path is responsive to said output signal and said combining meanscombines said first and second signal subtractively.

16. A signal processing system according to claim 11 comprising switchmeans for selecting compression or expansion operating mode of saidsystem, said main signal path including subtractive combining meansfollowed by additive combining means, said further signal path beingresponsive to the output of said subtractive combining means; and saidswitch means coupling said second signal to said additive combiningmeans for compression mode and to said subtractive combining means forexpansion mode.

17. A signal processing system according to claim 1 comprising switchmeans for selecting the compression or expansion operating mode of saidsystem, said switch means effecting signal connections whereby forcompression mode said further signal path is responsive to said outputsignal and said combining means combines said first and second signalsadditively and whereby for expansion mode said further signal path isresponsive to said input signal and said combining means combines firstand second signals subtractively.

18. A signal processing system according to claim 1 comprising switchmeans for selecting the compression or expansion operating mode of saidsystem, said main signal path including additive combining meansfollowed by substractive combining means, said further signal pathsbeing responsive to the output of said additive combining means; andsaid switch means coupling said second signal to said additive combiningmeans for compression mode and to said subtractive combining means forexpansion mode.

19. A signal processing system according toclaim 1 comprising switchmeans for disabling said further path, whereby the processing action ofsaid system can be disabled and said system transfers said input signalsto the output in a substantially proportional manner.

20. A signal processing system for producing an output signal inresponse to an input signal comprising a main signal path responsive tosaid input signal and including means for providing in a specifiedfrequency band a first signal substantially proportional to said inputsignal; a further signal path coupled to said main signal path andresponsive to a signal derived from said main signal path for producinga second signal in said specified frequency band, and signal combiningmeans for combining said first and second signals to produce said outputsignal, said further signal path including a first filter having fixedvalue components for providing high-pass characteristics with a cutofffrequency within said specified frequency band, and means for limitingthe amplitude of the output signal of the further signal path; saidmeans including a second filter connected in cascade with said firstfilter and having variable cutoff characteristics; said second filterincluding a variable impedance means responsive to an increase in theamplitude of at least one of the output of and the input to said furthersignal path for altering said cutoff characteristics from passingsubstantially all frequencies in said specified frequency band underlow-level input signal conditions to progressively passing only the highfrequencies therein, and providing a cutoff frequency which shiftsupwards sufficiently to attenuate, in combination with the saidhigh-pass characteristics of said first filter, any high-level inputsignal components at any given frequency in said specified frequencyband to a specified level corresponding in said second signal to a smallfractional part of the maximum level of said input signal, whilepermitting input signal components below said specified level at higherfrequencies than said given frequency to pass through said filters andto appear in said second signal with a lesser degree of attenuation.

21. A signal processing system according to claim [26] 20 comprisingmeans responsive to the output of said further signal path to controlthe impedance of said variable impedance means as a function offrequency such that said small fractional part decreases as said givenfrequency increases.

22. A signal processing system according to claim 20 wherein saidfurther signal path is responsive to said input signal; and wherein saidsignal combining means additively combines said first signal and saidsecond signal, whereby said signal processing system operates as acompressor.

23. In a tap recorder, for approximately correcting the high-frequencyemphasis introduced by a signal compressor in accordance with claim 22to the signal as recorded on a tape being played back by said recorder,the improvement consisting in a passive equalization network forapproximately correcting said emphasis, the network comprising a seriesresistance followed by a shunt arm which includes a resistor andcapacitor in series.

24. A signal processing system according to claim 20 wherein saidfurther signal path is responsive to said output signal; and whereinsaid signal combining means subtractively combines said second signaland said first signal, whereby said signal processing system operates asan expander.

25. A signal processing system according to claim 20 comprising switchmeans for selecting the compression or expansion operating mode of saidsystem; said main signal path including subtractive combining meansfollowed by additive combining means; said further signal path beingresponsive to the output of said subtractive combining means; and saidswitch means coupling said second signal to said additive combiningmeans for compression mode and to said substractive combining means forexpansion mode.

26. A signal processing system according to claim 20 comprising switchmeans for disabling said further path, whereby the processing action ofsaid system can be disabled and said system transfers said input signalto the output in a substantially proportional manner.

27. A method of processing an input signal to modify the dynamic rangethereof, wherein first and second signal components are derived inresponse to said input signal and combined to form an output signal;said first signal component being substantially proportional to saidinput signal within a frequency band extending between first and secondlimits; and said second signal component being subjected to the actionof a first filter having a fixed cutoff frequency lying between saidfirst and second limits and passing signals between said first limit andsaid cutoff frequency and the action of a second filter having avariable cutoff frequency which is shifted progressively away from saidsecond limit as said second signal component tends to increase, therebyto limit said second signal component to a small fractional part of saidfirst signal component when said input signal is at maximum level.

28. A method of processing an audio input signal to produce an outputsignal with reduced dynamic range comprising the steps of combininglinearly first and second signal components so that said secondcomponent boosts said first component above about 1.5 kHz. by about 10db at low signal levels to produce said output signal; providing saidfirst signal component in the form of a signal which has dynamic rangelinearity relative to said input signal; providing said second signalcomponent by filtering a signal which is derived from said input signalwith filter means having a high-pass characteristic above about 1.5 kHz.at low levels and by automatically narrowing the band pass of saidfiltering action to restrict, above a low-level threshold, said secondsignal component produced thereby to the high frequency part of theaudio frequency band and to a small fractional part of the maximumamplitude of said first signal component.

29. A method of processing an audio input signal to.

produce an output signal with increased dynamic range comprising thesteps of combining linearly first and second signal components so thatsaid second component bucks said first component above about 1.5 kHz. byabout 10 db at low signal levels to produce said output signal,

providing said first signal component in the form of a signal which hasdynamic range linearity relative to said input signal; providing saidsecond signal component by filtering a signal which is derived from saidoutput signal with filter means having a high-pass characteristic aboveabout 1.5 kHz. at low levels, and by automatically narrowing the bandpass of said filtering action to restrict, above a low-level threshold,said second signal component produced thereby to the high frequency partof the audio frequency band and to a small fractional part of themaximum amplitude of said first signal component.

30. A method of effecting noise reduction for an audio signal channelwherein an input signal is converted to an output signal, comprising thesteps of, providing a first signal component which, in the audiofrequency band, has dynamic range linearity relative to said inputsignal, providing a second signal component above about 1.5 kHz. byrestricting, above a low-level threshold, a signal derived from saidinput signal to a small fractional part of the maximum amplitude of saidfirst signal component, combining linearly said first and secondcomponents so that said second component boosts said first component toproduce a third signal, said second component increasing the level ofsaid third signal by about db. above about 1.5 kHz. at very low inputsignal levels, feeding said third signal to said channel and recoveringa fourth signal from said channel, providing a fifth signal componentwhich, within said band, has dynamic range linearity relative to saidfourth signal, providing a sixth signal component within said band byrestricting, above a lowlevel threshold, a signal derived from saidoutput signal to a small fractional part of the maximum amplitude ofsaid fifth signal component, combining linearly said fifth and sixthcomponents so that said sixth component bucks said fifth component toproduce said ouput signal, said sixth component decreasing the level ofsaid ouput signal by about 10 db. above about 1.5 kHz. at very lowfourth signal levels.

31. An audio noise reduction system for processing an input signal to aninformation signal from an information channel to produce a systemoutput signal substantially identical to said system input signal, saidsignal processing system comprising, a main signal circuit for providinga first signal component which, in the audio frequency band, has dynamicrange linearity relative to the signal applied to said main signalcircuit, a further signal circuit for providing a second signalcomponent within said band having a limited dynamic range relative tothe said signal applied to said main signal circuit, said further signalcircuit including filter means with a variable high-pass characteristicabove approximately 1.5 kHz. whereby the band pass of said filter meansnarrows to restrict, above a lowlevel threshold, said second signalcomponent to a small fractional part of said first signal component,means for combining linearly said first and second signal components,and switching means having compressor and expander modes, said switchingmeans being operative in the compressor mode to apply said input signalto said main signal circuit and to said further signal circuit and tocause said combining means to boost said first signal component by saidsecond signal component to the extent of approximately 10 db and to saidboosted first signal component as said information signal, saidswitching means being operative in the expander mode to apply saidinformation signal to said main signal circuit and said output signal tosaid further signal circuit and to cause said combining means to bucksaid first signal component by said second signal component to theextent of approximately 10 db and to provide said bucked first signalcomponent as said system output signal.

32. A method of compressing an input signal in the audio frequency bandto produce an output signal having a dynamic range decreased by about 10db at low levels within a restricted part of said audio frequency bandabove about 1.5 kHz., comprising the steps of. providin a circuit havingvariable frequency response characteristics over said restricted part ofsaid audio frequency band; supplying said input signal to said circuit,-deriving a control signal in response to signals in said circuit; andvarying said variable frequency response characteristics of said circuitin response to increasing amplitudes of said control signal so as tonarrow said restricted part of said audio frequency band.

33. A method in accordance with claim 32, wherein said control signal isderived from a signal appearing in said restricted part of said audiofrequency band, whereby said latter signal is excluded from saidnarrowed, restricted part of said audio frequency band.

34. A method in accordance with claim 32, and further including thesteps of rectifying and smoothing said control signal.

35. A method in accordance with claim 32, wherein said varying step inresponse to increasing amplitudes of said signals in said circuit abovea predetermined level.

36. A method of expanding an input signal in the audio frequency band toproduce an output signal having a dynamic range increased by about 10 dbat low levels within a restricted part of said audio frequency bandabove about 1.5 kHz., comprising the steps of; providing a circuithaving variable frequency response characteristics over said restrictedpart of said audio frequency band; supplying said input signal to saidcircuit; deriving a control signal from a signal appearing in saidrestricted part of said audio frequency band; varying said variablefrequency response characteristics of said circuit in response toincreasing amplitudes of. said control signal so as to narrow saidrestricted part of said audio frequency band, whereby said signalappearing in said restricted part of said audio frequency band isexcluded from said narrowed, restricted part of said audio frequencyband.

37. A method in accordance with claim 36, and further including thesteps of rectifying and smoothing said control signal.

38. A method of efiecting noise reduction by compressing a first inputsignal in the audio frequency band to produce a first output signalhaving a dynamic range decreased by about 10 db at low levels within arestricted part of said audio frequency band above about 1.5 kHz.,transferring said first output signal to provide a second input signal,and expanding said second input signal in said audio frequency band toproduce a second output signal having a dynamic range increased by about10 db at low levels within said restricted part of said audio frequencyband, said compressing comprising the steps of; providing a circuithaving variable frequency response characteristics over said restrictedpart of said audio frequency band; supplying said input signal to saidcircuit; deriving a control signal in response to signals in saidcircuit; and varying said variable frequency response characteristics ofsaid circuit in response to increasing amplitudes of said control signalso as to narrow said restricted part of said audio frequency band, andsaid expanding likewise comprising the steps of; providing a circuithaving variable frequency response characteristics over said restrictedpart of said audio frequency band; supplying said input signal to saidcircuit; deriving a control signal in response to signals in saidcircuit,- and varying said variable frequency response characteristicsof said circuit in response to increasing amplitudes of said controlsignal so as to narrow said restricted part of said audio frequencyband.

39. A method according to claim 38 wherein the said first output signalis recorded and played back to provide said second input signal, andwherein the compressing and the expending are efiected by the samecircuit switched to a compression configuration for compressing saidfirst input signal and to an expansion configuration for expanding saidsecond input signal.

40. A method according to claim 38, wherein said control si nal forcompressin and mid r'rmtrnl .cionnl mexpanding are each derived from asignal appearing in said restricted part of said audio frequency band,whereby said latter signal is excluded from said narrowed, restrictedpart of said audio frequency band.

41. A method in accordance with claim 38, wherein said varying steps arein response to increasing amplitudes of said signals in said circuitabove a predetermined level.

42. A signal processing system for producing an output signal inresponse to an input audio. signal, comprising:

(a) a main path signal responsive to said input signal a; provide a maincomponent, of said output signal, said main component having dynamicrange linearity with respect to said input signal;

(b) a further signal path input coupled to said main signal path toderive a signal'from said main signal path, said further path comprising(i) a variable high pass filter including a variable impedance meansresponsive to increase in the level of the signal being processed, atleast within the pass band of said variable high pass filter, to shiftthe low frequency cut off of said variable high pass filter in thedirection of increasing frequency,

(ii) means responsive to the output of said variable high pass filter tomodify the level of said main component in said main signal path,thereby to effect high audio frequency dynamic range modification ofsaid input signal, and

(iii) a further high pass filter having fixed characteristics whichcooperate with the characteristics of said variable high pass filter torestrict said modification of the level-"of said main component to theupper part of the audio frequency spectrum. I

43. A signal processing system according to claim 42,

wherein said modification of level of said main component is amodification ofabout db restribted to the part of the audio frequencyspectrum above about 1.5 Hz.

44. A signal processing system for producing an output signal in aspecifiedjrequency bandgin response to an input signal in said specifiedfrequency band, comprising:

(a) a first signal circuit responsive to said input signal to provide afirst component of said output signal, said first component havingdynamic range linearity with respect to said input signal;

(b) at least one linear combining means in said first signal circuit forcombining at least one further signal components with said first signalcomponent;

(c) a further signal circuit coupled to said first signal circuit andreceiving a signal from said first signal circuit, said further signalcircuit comprising:

(i) variable filter means with characteristics which respond to signallevels in the system (ii) at least one circuit means which respond tothe output of said filter means and operate with dynamic range linearityto produce the said at least one further signal components, whereby thesaid further signal components modify the dynamic range of said outputsignal with respect to said input signal in at least part of saidspecified frequency I band.

References Cited The following reference, cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

UNITED STATES PATENTS 3,111,635 11/1963 Skov et a1 333-14 3,350,51210/1967 Percival et a1. 33314 X 3,379,839 4/1948 Bennett 33314X3,397,285 8/1968 Golonski 33314 X ALFRED E. SMITH, Primary Examiner S.CHATMON, JR., Assistant Examiner US. Cl. X.R.

